This invention relates to a method of and apparatus for withdrawing a strand in a horizontal continuous casting installation in which a strand is intermittently withdrawn from a mold provided for a tundish by repeating a cycle of withdrawal of the strand over a predetermined stroke and retracting the strand over a small stroke.
The strand withdrawing device of the horizontal continuous casting installation is constructed to carry out cycles of intermittent withdrawal of the strand from a mold provided for a tundish during a predetermined number of strokes (10-20 mm, for example) and retraction of the strand over a small stroke (0.3-1.5 mm, for example).
Molten steel supplied from the tundish and cast in the mold starts solidification from its outer surface. Subsequently, when the solidified strand is withdrawn over a predetermined stroke, the molten steel is poured again into the mold, and the cast steel begins to solidify. The cast strand extending to the outside of the mold gradually cools and shrinks. For this reason, after the withdrawal, and after stopping the cast strand for a short waiting time the strand is retracted toward the mold over a distance or stroke at least equal to the amount of the shrinkage. The purpose of the retraction is to ensure positive connection between a newly solidified shell in the most upstream portion of the mold and a solidified shell contiguous thereto. Then, the cast strand is stopped for a very short interval, and then the withdrawal of the cast strand of the next cycle is started. By repeating such intermittent withdrawal and retraction at intervals of 0.5 second, for example, the strand is formed continuously.
The behavior of formation of solidified shell in the mold is disclosed in Japanese Laid-Open Patent Publication No. Hei-1-39,860 published in 1989. The small stroke of retracting the cast strand is very important for improving the quality of the strand. Where the retraction stroke is deficient, a cold shut cracks tend to be formed at the interface between solidified shells, whereas when the retraction stroke is too large an adverse phenomenon occurs in the steel structure during solidification, so that it becomes unable to obtain cast strands of a predetermined quality. Thus, the ability of the retracting device depends greatly upon the high degree of positional accuracy at the time of withdrawal and retraction of the cast strand.
The cast strand withdrawing device of the horizontal continuous casting installation disclosed in Japanese Laid-Open Patent Publication No. Sho-58-202,954 published in 1983 includes two sets of withdrawing devices each having a hydraulic type clamping mechanism for clamping the cast strand, and a withdrawal hydraulic cylinder for withdrawing the clamping mechanism. By alternately operating these withdrawing devices, the cast strand can be intermittingly withdrawn.
The cast strand withdrawing device of a horizontal continuous casting installation disclosed in Japanese Laid-Open Patent Publication No. Sho 54-24,224 published in 1979 comprises a reversible continuous rotating shaft system and a reversible intermittent rotating shaft system having a clutch, a reciprocating mechanism for converting a rotary motion into a reciprocating motion, a mechanism for forwardly rotating pinch rolls during a forward operation of the reciprocating mechanism, and a mechanism for rearwardly rotating the pinch rolls during a rearward operation of the reciprocating mechanism.
In the strand withdrawing device of Japanese Laid-Open Patent Publication No. Sho-58-202,954 referred to above, the strand is withdrawn by the hydraulic cylinder, so that there is an influence of the compressibility of the hydraulic fluid, and therefore the positional accuracy is small. Furthermore, the strand are alternately withdrawn with two sets of withdrawing devices, and the loads of the withdrawing devices are different when the withdrawing devices are changed, thus resulting in increase of the positional error.
In the strand withdrawing device disclosed in Japanese Laid-Open Patent Publication No. Sho-54-24,224 referred to above, the gear train of the intermittently reversed rotating shaft system has a large backlash, so that the positional accuracy of the strand is lowered greatly and therefore such device cannot be used practically. In addition, in this withdrawing device, the intermittently reversed rotating shaft system and the reversible continuously rotating shaft system protrude beyond the end surfaces of the pinch rolls, where a plurality of molds are mounted on the tundish. For this reason the spacing between the strands (spacing between the lines) becomes large, so that there is a problem in the installation space. Furthermore, the intermittently reversed rotating shaft system and the reversible continuously rotating shaft system include many parts (gears, shafts, clutches, sprocket wheels, chains, etc.) whereby the construction becomes complicated and bulky, and is expensive.